In most vehicles, their typical driving pattern involves frequent accelerations, each followed by a deceleration. Each acceleration involves a significant increase in fuel consumption needed to produce the additional energy necessary to increase the vehicle speed. Then, during a subsequent deceleration, this added energy is absorbed by vehicle brakes and dissipated as heat. Attempts to overcome such waste of energy led to development of systems in which the energy of vehicle motion is not dissipated during braking, but converted into a form in which it can be temporarily stored and, then, used again to accelerate the vehicle at a later time. An electric hybrid is a typical and currently prevailing example of such system.
An electric hybrid vehicle has two power plants, an internal combustion engine and an electric generator/motor. It also includes an electric battery for energy storage. During braking, the generator/motor absorbs the energy of vehicle motion and deposits it into the battery. During subsequent acceleration, the generator/motor receives energy from the battery and uses it to supplement the work of the internal combustion engine. A disadvantage, common to all types of electric hybrids, is the added cost and complexity associated with the need for the additional electric power plant and associated mechanisms needed to connect it to the vehicle wheels in-parallel to or in-line with the internal combustion engine. Added complexity also increases probability of failures, thus contributing to a reduction in overall system reliability. Another significant disadvantage is the weight of the battery. Electric batteries are excellent energy storage devices, but their weight adds substantially to the mass of the vehicle.
To alleviate the issue of high cost and complexity, a concept of air hybrid was developed. In an air hybrid, there is no need for a second power plant. The internal combustion engine itself absorbs the vehicle braking energy by operating as a compressor, during braking, and deposits the compressed air into an air tank for storage. Then, during acceleration, the energy of compressed air assists the engine in propelling the vehicle. The air-hybrid concept was described in U.S. Pat. No. 6,223,846B1. The above patent describes an air-hybrid engine that employs an electro-hydraulic system to operate and control the engine valves. The engine has no camshafts. Instead, each valve is equipped with a double-acting piston that, when exposed to high-pressure fluid, causes the valve to move. The system includes a set of high-speed solenoid valves that control the timing of the engine valves opening and closing. Two solenoid valves are required for each engine valve—one to control the timing of valve opening, and a second one to control the timing of closing. Varying the timing of the engine valve opening and closing varies the valve opening duration.
The air-hybrid system, described in the above patent, is considerably simpler and less expensive than an electric hybrid. Still, its cost is much higher than the cost of a conventional engine equipped with a camshaft-driven valve train. Most of the cost penalty is associated with the need for a high-pressure hydraulic system and a large number of solenoid valves. In addition, any camless valve train has a potential reliability problem associated with the fact that a failure of a key electronic component, such as a solenoid valve, may cause a piston-to-valve collision. Such collision is likely to result in engine failure. Also, the need to convert the engine mechanical energy into hydraulic energy, to drive the valves, and into electric energy, to drive the solenoid valves, is an inefficient process that is detrimental to the system efficiency.
A conventional engine, equipped with a camshaft-driven valve train, operates its valves without high-pressure hydraulics and without solenoid valves. A camshaft is a relatively low friction device that requires very little of the engine energy for its operation. As long as mechanical integrity of the valve train components is retained, there is no possibility of piston-to-valve collision, because both piston and camshaft are mechanically driven from the engine crankshaft, and the geometry of the cam lobes driving the valves prevents such collision.
In view of the above, it is clear that, to achieve further improvement in air-hybrid cost, efficiency, and reliability, it is highly desirable to have an air-hybrid system using camshafts for engine valve operation. Such a system is the subject of the present invention.